I work on a market making team at a major investment bank, and I can back this up. There is a very clear and serious separation between those who trade bank funds and those who trade client funds. We are not allowed to know anything about their positions, accounts, strategies, etc. (and vice versa).

Not only are there very stringent access controls in place to prevent cross-contamination, but there are physical boundaries as well. When the guy who runs the business side of our team started collaborating on a project with a team that did client trading, he had to move off our trading floor and into an office, and we had to revoke this access to, well, pretty much everything. He wasn’t allowed to go near our traders’ desks. It stayed that way until the project ended.

Just going to second this. I used to work for Virtu, which bought Getco and the smoldering pile of ashes left from Knight Capital’s meltdown almost solely for their Designated Market Maker status for so many symbols.

There is a miscommunication here, arising from the fact that bunderbunder thinks that 'Citigroup' is an entity, and is clearly both a market maker and has clients, whereas you (and other professionals in the field) think 'Citigroup's equity option market making desk' and 'Citigroup's prime brokerage division' are entities, and are clearly only one of those things each.

People at a distance from organisations see them as monolithic. People closer to them see them as coalitions of distinct entities.